Page 354 - Advanced Mine Ventilation

Economics of Coal Mine Degasification 331

Table 20.2 Coalbed Gas Composition

Typical
Component Analysis (%) Range (%)
Methane 95 85e96
Ethane 0.5 0.2e2.5
Propane Trace Trace
Isobutane Trace Trace
Normal butane Trace Trace
Isopentane Trace Trace
Nitrogen 1.5 1e6
Carbon dioxide 3 0.5e10

specifications vary, but in general, they limit the percentage of noncombustibles to less
3
than 4% and a minimum BTU of 960/ft .
The produced gas has to be gathered and processed before it can be sold.

20.4.3.1 Gathering Gases

Several CBM-producing wells are connected to a small compressor that feeds into a
bigger compressor or the main compressor at the processing plant. These compressors
can be run by gas engines, but it is preferable to have electrical drives for reliability.
Most often HDPE (high-density polyethylene) lines are used for gathering gases. The
line pressure is typically less than 50 psi.

20.4.3.2 Gas Processing
The raw gas is seldom of pipeline quality. Most often, it is processed to meet the mar-
keting pipeline specifications.
The general scheme is to pass the raw gas through “intermediate” compressors
raising the pressure to 200e300 psi.
The gas processing line diagram is shown in Fig. 20.1.
The compressed gas first goes through a cyclone where free water and solids are
removed. Next, it goes in a chamber where oxygen is removed by burning some
methane catalytically. The gas goes through a “diamine” unit next where all CO 2 is
removed. Next, the gas goes through a dehydration unit that contains ethylene glycol.
The gas is now ready for commercial pipelines unless there is more than 4% nitrogen.
In this case, a nitrogen rejection unit (NRU) is used to remove most of nitrogen in the
gas. There are two kinds of NRU: (1) the molecular gate process and (2) the cryogenic
process. In the molecular gate system, unique molecular sieves have pore-size open-

ings that can be controlled with a precision of 0.1 A . The pore size is precisely